Hepatitis B is a major healthcare problem throughout both the developed and developing world. Infection with the hepatitis B virus (HBV) can result in an acute or chronic disease which in a proportion of cases may lead to hepatocellular carcinoma and death. The virus is double shelled, and its DNA is protected inside a protein structure called the core antigen (HBcAg). The core is surrounded by the envelope protein known as the surface or S antigen (HBsAg). HBcAg is an unusual antigen which can be used as a delivery vehicle for specific peptides to the immune system. The antigen has been used to present T-helper, B and cytotoxic lymphocyte (CTL) epitopes from a variety of viral and bacterial pathogens, including epitopes from the surface antigen of HBV, envelope proteins from hepatitis A virus and antigens from hepatitis C virus. For a review see Ulrich et al (1998) Advances in Virus Research 50 141-182.
HBcAg is an excellent vehicle for the presentation of epitopes due to the molecular structure of the protein, which self-assembles into particles. Each particle is generated from either 180 or 240 copies of a monomeric polypeptide. The monomer, on reaching an appropriate concentration inside the host cell, forms a particle of approximately 27 nm in diameter. Structural studies have shown that amino acids within the region from residues 68 to 90 form a spiked structure on the surface of the particle which is known as the e1 loop. Two monomers joined by disulphide bonds link to form a dimer spike, the most exposed amino acid being at position 80 (at the centre of the e1 loop).
EP-A421635 (The Wellcome Foundation Limited) describes modification of the HBV core gene to allow insertion of foreign epitopes into the e1 loop without altering the potential of the protein to from particles. Insertion at this site allows maximum exposure of the inserted epitope on the tip of each spike created by dimers of the protein. As there are approximately 180 or 240 copies of each monomer per particle, each particle is able to present 180 or 240 copies of the epitope of interest